This invention relates to multilamp photoflash units having circuit means for igniting of the flashlamps, and, more particularly, to high voltage photoflash arrays with improved means for providing electrostatic protection.
Numerous multilamp photoflash arrangements with various types of sequencing circuits have been described in the prior art, particularly in the past few years. A currently marketed photoflash unit (described in U.S. Pat. Nos. 3,894,226 and 4,017,728 and referred to as flip-flash) employs high voltage type lamps adapted to be ignited sequentially by successively applied high voltage firing pulses (e.g., 2,000 to 4,000 volts) from a source such as a camera-shutter-actuated piezoelectric element. The flip-flash unit comprises an elongated planar array of eight high voltage type flashlamps mounted on a printed circuit board with an array of respectively associated reflectors disposed therebetween. The construction further includes the front and back plastic housing members with interlocking means for providing a unitary structure. The front housing member is a rectangular concavity, and the back housing is substantially flat. Sandwiched between the front and back housing members, in the order named, are the flashlamps, a unitary member, preferably of aluminum-coated plastic, shaped to provide the eight individual reflectors of the array, an insulating sheet, a printed circuit board, and an indicia sheet, which is provided with information, trademarks and flash indicators located behind the representative lamps which change color due to heat and/or light radiation from a flashing lamp, thus indicating at a glance which of the lamps have been flashed and not flashed.
The primers used in the high voltage type flashlamps employed in such arrays are designed to be highly sensitive toward high voltage breakdown. Electrical energies as low as a few microjoules are sufficient to promote ignition of such primers and flashing of the lamps. The high sensitivity is needed in order to provide lamps that will function reliably from the compact and inexpensive piezoelectric sources that are practical for incorporation into modern miniature cameras.
The high degree of electrical sensitivity needed in high voltage flashlamps gives rise to distinct problems of inadvertent flashing during handling of the array package. Any static charges on equipment and personnel can cause the lamps to flash. This problem is discussed in U.S. Pat. No. 3,935,442, and one means described therein for protecting against inadvertent flashing is to make the reflector member electrically conductive, such as fabricating it of metal or metal-coated plastic, and electrically connecting the reflector to an electrical "ground" portion of the circuitry on the circuit board. Thus, the reflector member functions as an electrical shield and increases the stray capacitance to ground of the electrical "ground" of the circuitry, reducing the possibility of the accidental flashing of the lamps by electrostatic voltage charges on a person or object touching the array.
A further approach, described in U.S. Pat. No. 3,941,992, for providing electrostatic protection is to metalize the back surface of the circuit board and connect that metalized surface to the common circuit conductor run, for example, by means of an eyelet through the board, thereby providing a planar conductive shield behind the lamps and circuitry. An alternative approach for providing a rear shield is described in U.S. Pat. No. 4,019,043, wherein a conductive shield of planar configuration is spaced from the back surface of the circuit board and connected to the common circuit conductor. In particular, this shield is described as comprising a sheet of metallic foil laminated to the indicia sheet.
Other patents relating to electrostatic protection for this general type of photoflash array include U.S. Pat. Nos. 3,980,875; 3,980,876; 4,093,979; 4,104,705; 4,104,706; 4,133,023; 4,136,375; and 4,060,721.
Improved approaches for enhancing the electrostatic protection provided by the aforementioned shielding techniques are described in other patents. For example, U.S. Pat. No. 3,980,877 employs one or more openings through the housing wall adjacent to an electrical ground part of the circuitry to provide a discharge path therethrough for a person or object touching the array. U.S. Pat. No. 4,041,300 describes the use of a plastic housing with a surface coating of electrically conductive material and one or more openings through the housing wall to provide a discharge path. And in U.S. Pat. No. 4,113,424, a plurality of conductive posts project from the grounded reflector array and protrude through respective openings in the light-transmitting front face of the housing.
A copending patent application Ser. No. 72,251, assigned to the present assignee, describes a more compact, cost-efficient photoflash unit construction comprising a linear array of electrically ignitable flashlamps mounted on a printed circuit board in the form of an elongated strip. The printed circuit board is located within the longitudinal channel of an elongated housing member having a continuous coating of conductive reflective material thereon adjacent to the lamps. A light-transmitting cover panel is attached to the front of the housing member to enclose the flashlamps. The lamps have substantially tubular envelopes and are positioned extremely close to one another with their longitudinal axes substantially parallel to the surface of the printed circuit strip and in substantially coaxial alignment. Typically, the diameter of the lamps, the width of the printed circuit strip and the width of the channel in the housing member are nearly equal. The circuitry on the circuit board includes one conductor run which is connected in common to a lead-in wire of each of the lamps of the array. Means, such as a segment of a soldered wire or an edge deposit of conductive material, is disposed between the common circuit conductor run and the channel wall of the housing in contact with the conductive-reflective coating thereon. In this manner, the conductive wall coating is grounded to common to provide an electrostatic shield for the unit. Protection against electrostatic charges about the front face of the unit is provided in a manner following teachings of the aforementioned U.S. Pat. No. 4,133,424. More specifically, the light-transmitting cover panel is provided with a plurality of openings, and conductive projections in the housing member protrude through the cover openings to extend the circuit ground to the exterior of the front face of the unit.
The aforementioned compact linear array construction, however, does not provide such an exterior ground mode of protection for the rear of the unit. Accordingly, the present invention is particularly directed to the object of providing enhanced electrostatic protection at the rear of the unit. This poses a problem, however, as the aforementioned approaches employed with the more complex photoflash structures, including the use of separate shields of metalic foil, are not totally satisfactory or compatible with the compactness, simplicity, and cost-effective characteristics, of the aforementioned linear photoflash array, wherein the unit housing provides the additional functions of multicavity reflector and electrostatic shield for the circuit strip-lamp assembly mounted internally thereof.